Use of a compound based on a rare-earth phosphate as a luminophor in plasma systems

ABSTRACT

The invention relates to the use as a luminophor in plasma systems of a compound based on an yttrium, lanthanum, gadolinium or lutecium phosphate doped with at least one rare earth chosen from the group comprising terbium, praseodymium, europium and cerium.

This application is a continuation of application Ser. No. 08/672,523filed Jun. 25, 1996 which application is now: abandoned now U.S. Pat.No. 5,725,800.

The present invention relates to the use of a compound based on arare-earth phosphate as a luminophor in plasma systems.

Plasma systems (screens and lamps) form part of new display and lightingtechniques which are being developed. One concrete example consists inthe replacement of current television screens by flat screens, which areless heavy and have larger dimensions, and this replacement is on thepoint of being resolved by the use of plasma panels.

In plasma systems, a gas introduced into a chamber is ionized under theeffect of an electric discharge. High-energy electromagnetic radiationis emitted during this process. The photons are directed onto aluminescent material.

In order to be effective, this material should be a luminophor whichabsorbs in the emission range of the plasma and emits in the visiblerange with the highest possible efficiency and with the appropriatecolour.

The object of the invention is to provide such a luminophor material.

To this end, according to the invention, the material used as aluminophor in plasma systems is a compound based on an yttrium,lanthanum, gadolinium or lutecium phosphate doped with at least one rareearth chosen from the group comprising terbium, praseodymium, europiumand cerium.

Other characteristics, details and advantages of the invention willbecome more clearly apparent on reading the following description.

The invention relates to the use of the compound described above as aluminophor under conditions corresponding to those in plasma systems. Inthe context of the present invention, this term is taken to mean allsystems employing a gas which, after ionization, emits radiationcorresponding at least to wavelengths between 10 and 200 nm, that is tosay the far ultraviolet range.

Systems of this type which may be mentioned include plasma lamps andscreens.

According to the invention, use is made of a compound comprising amatrix of the yttrium, lanthanum, gadolinium or lutecium phosphate type.

Several types of phosphates may be used. They may be orthophosphates offormula LnPO₄, Ln representing one of the elements mentioned above.Metaphosphates of formula LnP₃ O₉ or pentaphosphates of formula LnP₅ O₁₄may also be used.

The matrix of the type described above is doped with at least one rareearth which is chosen from the group comprising terbium, praseodymium,europium and cerium.

The rare earth is chosen in accordance with the desired emission colour.

The rare-earth dopant content, expressed as an atomic percentagerelative to the total rare-earth content in the phosphate (rare-earthdopant/rare-earth dopant+Ln), is generally between 10 and 50%, and moreparticularly between 20 and 45%.

According to a particular embodiment of the invention, the phosphateused is a lanthanum phosphate.

According to another particular embodiment of the invention, the rarecarth dopant is a combinaison of terbium and cerium. The terbium may bein a major proportion with respect to the cerium and more particularlyin an atomic proportion Tb/Ce+Tb of at least 80%. As a example theproduct of the following formula can be given

La₀.76 Tb₀.22 PO₄

According to another variant of the invention, it may be beneficial touse phosphates with specific particle size distributions.

The mean size of the phosphates may thus be between 1 and 20 μm, andmore particularly between 2 and 6 μm.

Furthermore, their dispersion index may be less than 0.5, and moreparticularly less than 0.4.

The mean diameter of the particles is measured by using a lasergranulometer of the CILAS type (CILAS HR 850).

For its part, the dispersion index I is determined by the formula:##EQU1## in which: φ₈₄ is the particle diameter for which 84% of theparticles have a diameter less than φ₈₄ ,

φ₁₆ is the particle diameter for which 16% of the particles have adiameter less than φ₁₆ ,

and φ₅₀ is the mean diameter of the particles.

The phosphates used and, more particularly, the orthophosphates, may bein a monoclinic crystalline form.

The phosphates used in the context of the present invention can beobtained by any known method. One particular method, given withoutimplying any limitation, will be described below.

This method consists in carrying out direct precipitation at controlledpH by reacting (I) a first solution containing soluble salts of rareearths, these elements then being present in the stoichiometricproportions required for obtaining a product with the desired formula,with (II) a second solution containing phosphate ions.

The solution of soluble rare-earth salts is progressively andcontinuously introduced into the solution containing the phosphate ions.

The initial pH of the solution containing the phosphate ions is lessthan 2, and preferably between 1 and 2.

The pH of the precipitation medium is subsequently adjusted to a pHvalue of less than 2, and preferably between 1 and 2.

The term "controlled pH" means that the pH of the precipitation mediumis kept at a certain constant, or substantially constant, value byadding basic compounds or buffer solutions into the solution containingthe phosphate ions, at the same time as the solution containing thesoluble rare-earth salts is introduced into this solution. The pH of themedium will thus vary by at most 0.5 of a pH unit around the fixedset-point value, and more preferably by at most 0.1 of a pH unit aroundthis value. The fixed set-point value will advantageously correspond tothe initial pH (less than 2) of the solution containing the phosphateions.

The pH is advantageously adjusted by adding a basic compound, as will beexplained below.

The precipitation is preferably carried out in an aqueous medium at atemperature which is not a critical factor and which is advantageouslybetween room temperature (15° C.-25° C.) and 100° C. This precipitationtakes place with the reaction medium being stirred.

The concentrations of the rare-earth salts in the first solution canvary within wide limits. Thus, the total rare-earth concentration may bebetween 0.01 mol/liter and 3 mol/liter.

Suitable rare-earth salts are, in particular, salts which are soluble inan aqueous medium, such as, for example, nitrates, chlorides, acetates,carboxylates or a mixture thereof. The salts which are preferredaccording to the invention are nitrates.

The phosphate ions intended to react with the rare-earth salt solutionmay be supplied by pure compounds or compounds in solution, such as, forexample, phosphoric acid, phosphates of alkali metals or other metallicelements which give a soluble compound with the anions associated withthe rare earths.

According to a preferred variant, the phosphate ions are added in theform of ammonium phosphates because the ammonium cation will decomposeduring the subsequent calcination, thus making it possible to obtain ahigh-purity rare-earth phosphate. Among the ammonium phosphates,diammonium or monoammonium phosphate are the preferred compounds forimplementing the invention.

The phosphate ions are present in a quantity such that there is, betweenthe two solutions, a PO₄ /Ln molar ratio of greater than 1, andadvantageously between 1.1 and 3.

As already indicated, the solution containing the phosphate ions shouldinitially have (that is to say before the start of the introduction ofthe rare-earth salt solution) a pH of less than 2, and preferablybetween 1 and 2. Thus, if the solution used does not naturally have sucha pH, the pH is brought to the desired suitable value either by adding abase (for example ammonia solution in the case of a phosphoric acidinitial solution) or by adding an acid (for example nitric acid, in thecase of a diammonium phosphate initial solution).

Subsequently, as the solution containing the rare-earth salt or salts isintroduced, the pH of the precipitation medium decreases progressively.A base is thus simultaneously introduced into this medium, with the aimof keeping the pH of the precipitation medium at the desired constantworking value, which should be less than 2 and preferably between 1 and2.

Suitable basic compounds which may be mentioned by way of examples aremetal hydroxides (NaOH, KOH, CaOH₂, . . . ) or ammonium hydroxide, orany other basic compound whose constituent species will not form aprecipitate when they are added to the reaction medium, by combiningwith one of the species also contained in this medium, and which allowsthe pH of the precipitation medium to be adjusted.

At the end of the precipitation step, a phosphate precipitate isdirectly obtained which can be recovered by any means known per se, inparticular by simple filtration. The recovered product can then bewashed, for example with water, with the aim of removing possibleimpurities from it, in particular adsorbed nitrate and/or ammoniumgroups.

The precipitate obtained is then subjected to a heat-treatment at atemperature generally above 600° C. and advantageously between 900 and1200° C. This heat-treatment or calcination makes it possible to obtaina product which has luminescence properties. It is equally well possibleto carry out the calcination under a reducing atmosphere (for examplehydrogen) or a neutral atmosphere (for example argon), or mixturesthereof or else under a non-reducing atmosphere, in particular under anoxidizing atmosphere such as, for example, air.

The phosphates used in the context of the present invention can also beobtained by the chamotte process. In this case, the procedure adoptedmay be to form a mixture of the oxides of the various rare earths or totake a mixed rare-earth oxide and phosphatize this mixture or this mixedoxide by calcination in the presence of diammonium phosphate.

In order to develop the luminescence properties further, the phosphatesmay be subjected to a heat-treatment in the presence of a flux.

Suitable fluxes which may be mentioned are, in particular, lithiumfluoride, lithium chloride, potassium chloride, ammonium chloride, boronoxide and ammonium phosphates, this list being, of course, in no waylimiting. The flux is mixed with the mixed phosphate to be treated, thenthe mixture is heated to a temperature of greater than 1000° C.,generally between 1000° C. and 1200° C., this being done under anatmosphere which is suited to the nature of the rare earth and which isa reducing atmosphere in the case of cerium or terbium, for example.After treatment, the product is washed then rinsed, so as to obtain aluminophor in a deagglomerated state.

As indicated above, the phosphate-based compounds which have just beendescribed have properties of luminescence under electromagneticexcitation in the wavelength range used in plasma systems.

For this reason, they can be used as a luminophor in these systems and,more generally, they may be incorporated in the manufacture of suchsystems. The luminophors are employed according to well-knowntechniques, for example deposition by screen printing, electrophoresisor sedimentation.

An example is given below.

EXAMPLE

Preparation of the phosphates

The preparation of the product of formula La₀.55 Ce₀.30 Te₀.15 PO₄ isgiven above. The other phosphates, of formula La₀.88 Tb₀.12 PO₄, La₀.83Tb₀.17 PO₄, La₀.70 Tb₀.30 PO₄ and La₀.78 Tb₀.22 PO₄, are prepared in thesame way by adapting the reactant proportions.

500 ml of a solution of rare-earth nitrates with an overallconcentration of 1.5 mol/l and having the following composition: 0.825mol/l of La(NO₃)₃ ; 0.45 mol/l of Ce(NO₃)₃ and 0.225 mol/l of Tb(NO₃)₃,are added in one hour to 500 ml of a phosphoric acid solution H₃ PO₄,previously brought to pH 1.4 by adding ammonium solution and heated to60° C.

The phosphate/rare-earth molar ratio is 1.15. The pH during theprecipitation is adjusted to 1.4 by adding ammonia solution.

After the precipitation step, the reaction medium is further maintainedat 60° C. for one hour.

The precipitate is then recovered by filtration, washed with water thendried at 60° C. under air. The product is then in the form of a whitepowder (resembling a talc) consisting of particles (agglomerates) of 3to 6 microns formed by compact aggregates of approximately 250 nm,themselves formed by the aggregation of elementary crystallites withsizes of between 30 and 150 nm. The powder is then subjected to aheat-treatment at 1150° C. under air.

X-ray analysis shows that the product is an orthophosphate withmonoclinic crystalline structure. It consists of compact grains ofapproximately 250 nm, agglomerated in the form of spherical particleswith sizes of between 3 and 6 microns. The CILAS particle sizedistribution, measured after cursory deagglomeration under ultrasound,gives a φ₅₀ of 4.5 microns with a very tight distribution since thedispersion index is less than 0.4.

Performance of the phosphates

The performance is evaluated in two ways.

A--the powder is compacted by hand in cylindrical cavities, 8 mm indiameter, which are then arranged in a chamber under a vacuum of 10⁻⁶torr at room temperature. The excitation source is electromagneticemission produced by a synchrotron and with a wavelength of between 150and 350 nm. The efficiency values correspond to the number of photonsemitted in the visible range compared with the number of incidentphotons at 160 nm and 200 nm, respectively.

    ______________________________________                    Efficiency % at                               Efficiency % at    Product         200 nm     160 nm    ______________________________________    La.sub.0.88 Tb.sub.0.12 PO.sub.4                    60         72    La.sub.0.83 Tb.sub.0.17 PO.sub.4                    65         76    La.sub.0.78 Tb.sub.0.22 PO.sub.4                    70         85    La.sub.0.55 Ce.sub.0.30 Tb.sub.0.15 PO.sub.4                    70         85    ______________________________________

E--The products are evaluated on a plasma-screen test cell containing agas of the helium-neon type. The light output efficiency is measuredwith a photometer and it is compared with that of manganese-doped zincsilicate, to which a value of 100 is arbitrarily assigned.

    ______________________________________    Product       Light output efficiency    ______________________________________    La.sub.0.83 Tb.sub.0.17 PO.sub.4                  125    La.sub.0.78 Tb.sub.0.22 PO.sub.4                  134    La.sub.0.70 Tb.sub.0.30 PO.sub.4                  120    ______________________________________

It is seen that the light output efficiencies are superior to thatobtained with the doped zinc silicate used in the prior art.

I claim:
 1. A process for having a plasma system emit wavelengths in thevisible range comprising directing photons of a wavelength between 10and 200 nm, from an ionized gas onto a luminophor comprising:(i) acompound based on a phosphate of a rare-earth of the formula LnP₃ O₉ orLnP₅ O₁₄, Ln being selected from the group consisting of yttrium,lanthanum, gadolinium or lutecium, and (ii) a dopant of compound (i)being a rare-earth which is a combination of terbium, and cerium.
 2. Aprocess according to claim 1, wherein the phosphate has a rare-earthdopant content, expressed as an atomic percentage, relative to the totalrare-earth content of the phosphate, of between about 10% and about 50%.3. A process according to claim 1, wherein the atomic proportionterbium/cerium+terbium is of at least about 80%.
 4. A process accordingto claim 1, wherein the phosphate has a mean particle size of betweenabout 1 and about 20 μm and a dispersion index of less than about 0.5.